The membranes of both purified mast cells and mastocytoma cells have been studied using spin-labeled probes. The membrane fluidity of the neoplastic cells was significantly greater than that of the normal mast cells. The character of the spin probe was also found to be important in determining the accuracy of fluidity studies. Highly charged probes (such as fatty acids) may report a lower apparent fluidity due to the anchoring influence of their charged end group. Compound 48/80, a mast cell degranulating agent, had no effect on any of the lipid spin labels studied; however, it was able to reverse the effect of magnetic interations between closely adjacent spin labels. This suggests that 48/80 may increase the available membrane binding sites for the spin labels. Compound 48/80 also affected the state of MSL-ghost proteins by causing an increase in the population of highly immobilized label. The effect of trypsin was similar between MSL-ghosts and SL-48/80-labeled mast cells. Fluorescence microscopy has shown that mast cells and mastocytoma cells bind R-48/80 on the plasma membrane at low concentration and at high concentration, the binding also occurred at anionic sites in the cell interior.